An exemplary system for circulating a slurry or liquid mixture to disperse substances in a mixture is disclosed in Japanese Patent Laid-open Publication No. 2004-267991. In a system for dispersing by circulation like this, the following must be considered in order to use a rotating and continuously dispersing device. That is, in such a device, an O ring, an oil seal, a gland packing, a mechanical seal, or the like is used for a part for sealing a shaft that prevents the leakage of the mixture through the axis. When preventing the leakage of a slurry that has a high concentration, such as a concentration of solids over 40 to 50%, often a mechanical seal is used.
However, there are problems in using a mechanical seal. The structure of it is complicated, the seal is large, and its cost is high. Moreover, if a mixture penetrates the seal and particulates enter there, the sealing surface (shaft seal side) is damaged, to deteriorate the performance. Therefore, there is also a problem in that a double mechanical seal, which is especially expensive and has a more complicated structure, will be needed.